1. Field of the Invention
The present invention relates to a method and a device for controlling an object to be controlled, and more particularly to a set up control method and a set up control device for deciding a set point of a nonlinear control system such as a rolling mill.
2. Description of the Related Art
It is possible to carry out a high-precision control of a set up for deciding a set point of a control system when a linear control theory which has been developed based on a linear control system is used. However, even if this linear control theory is applied to an object of strong nonlinearity to be controlled such as, for example, a rolling mill for rolling a material, it is not possible to expect a desired level of control performance and it is not possible to improve the quality of the product (a steel material in the case of the rolling mill) to a high level. As a control system for controlling a rolling mill, a set up control system is available for obtaining a set point in a steady state when there is no time change in the controlled object, and a method for obtaining a high-precision control of the set up control system has been desired. This set up control is referred to as a set point control in the control of a thermoelectric power generator and a plant control.
Usually, in a set up control system, an optimal solution is obtained based on a dynamic programming or linear programming method, by disregarding a time change factor, in the same manner as when a stationary solution of a differential equation is obtained, thus determining a set point. Once the set point has been obtained, feedback control is carried out based on a linear control theory, according to which an object to be controlled is linearized around the set point to set an error from the set point to zero, as a regulator, and an optimal solution is obtained for a transient solution of the differential equation.
The set up control is described in detail in "The Theory and Practice of a Plate Rolling" by The Iron and Steel Institute of Japan, issued on Sep. 1, 1984, pp. 289-292.
In the above-described prior-art technique, it has been always the case that a control model for deciding a set point for a set up control (or a set point control) always changes. In the case of a rolling mill, for example, there are too many unknown parameters. Further, even if the value of each parameter has been decided it is usually the case that the value of each parameter, once decided, is easily changed due to changes in environmental factors, such as, for example, the value of the frictional resistance changes and the roll expands due to heat generated during the rolling operation. Since it is difficult to accurately decide changed values of parameters, the set point once decided by the set up control changes greatly and thus parameters of the feedback control also change, with a result that it is impossible to obtain a product (a steel product in the case of the rolling mill) of satisfactory quality.